Thermal paste for improving thermal contacts

ABSTRACT

A thermally conductive paste including porous agglomerates of carbon particles dispersed in a paste-forming vehicle is disclosed. The paste is useful as a thermally conductive interface material between a heat or cold source and an object. The paste is particularly useful as a thermally conductive interface material between a heat source and a heat sink. Apparatus and a method of removing heat from a heat source utilizing thermally conductive pastes of the present invention are also disclosed.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/485,804, filed Jul. 9, 2003, which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a thermally conductive paste for improvingthermal contacts.

BACKGROUND OF THE INVENTION

With the miniaturization and increasing power of microelectronics, heatdissipation has become critical to the performance, reliability andfurther miniaturization of microelectronics. Heat dissipation frommicroelectronics is most commonly performed by thermal conduction. Forthis purpose, a heat sink, which is a material of high thermalconductivity, is commonly used. In order for the heat sink to be wellutilized, the thermal contact between the heat sink and the heat source(e.g., a substrate with a semiconductor chip on it) should be good.Wolff et al., Heat & Mass Transfer 41 :3469-3482.(1998); Ouellette etal., Proc. Power Elec. Des. Con., Power Sources Users Conf., Cerritos,Calif., pages 134-138 (1985).

A thermal fluid or paste is commonly applied at the interface to enhancethe thermal contact. Vogel, Proc. Int. Intersoc. Elec. Pkg. Conf, Adv.in Elec. Pkg., Am. Soc. Mech. Eng., NY, N.Y. 10-2:989 (1995). The fluidor paste is a material that has high conformability so that it canconform to the surface topography of the mating surfaces, therebyavoiding air gaps (which are thermally insulating) at the interface. Thefluid or paste must be highly spreadable, so that the thickness of thepaste after application is very thin (just enough to fill the valleys inthe surface topography of the mating surfaces). Preferably the fluid orpaste is thermally conductive as well. Although much attention has beengiven to the development of heat sink materials, relatively littleattention has been given to the development of thermal fluids or pastes.

The most common thermal fluid is mineral oil. As a fluid, it is highlyconformable and spreadable, but it has a low thermal conductivity. Themost common thermal paste is silicone filled with thermally conductiveparticles. Wilson et al., Nat. Elec. Pkg. & Prod. Conf, Proc. Tech.Prog., Reed Exh. Co., Norwalk, Conn. 2:788-796 (1996); Peterson, Proc.40th Elec. Comp. & Tech. Conf., IEEE, Piscataway, N.J. 1:613-619 (1990);Lu et al., J Polym. Sci., Part B 36:2259-2265 (1998); Sasaski et al.,Jap. IEMT Symp. Proc., IEEE/CPMT Int. Elec. Mfg. Tech. Symp., IEEE,Piscataway, N.J. 236-239(1995). Due to the filler, it is relatively highin thermal conductivity, but it suffers from poor conformability andpoor spreadability. Thermal fluids and pastes of previous work are notas effective as solder (applied when it is molten), but they do notrequire heating, which is required for the use of solder. Xu et al., J.Electron. Pkg. 124:188-191 (2002); Xu et al., J. Electron. Pkg.122:128-131 (2000).

Due to its excellent heat transfer characteristics and as it isrelatively inexpensive, boron nitride is commonly used as a filler forthermal interface materials. Unfortunately, however, it suffers from thedisadvantage that it degrades when exposed to humidity. When placed in ahumid environment, hygroscopic impurities (boric oxide) within thecompound absorb atmospheric water, which then reacts with the boronnitride to form boric acid. Being hygroscopic, the boric acid absorbsfurther water, thereby accelerating the degradation of the boron nitrideand diminishing its heat removing capabilities, which ultimately leadsto failure of the device. Published PCT Application WO 01/21393 isspecifically directed to this problem and describes a moistureresistant, thermally conductive material that includes thermallyconductive filler particles, preferably boron nitride, that are coatedwith a hydrophobic compound, preferably a silicone compound such as asiloxane. The hydrophobic compound-coated filler particles are joinedtogether with a binder, and account for between 5 and 70 vol. % of thematerial.

Organic vehicles are commonly used as the suspending medium fordispersed inorganic particles in pastes. Kumar, Active & Passive Elec.Comp. 25:169-179(2002); Chae et al., Mater. Lett. 55:211-216 (2002);Heller et al., Tenside, Surfactants, Detergents 29:315-319 (1992);Stanton, Int. J. Hybrid. Microelec. 6:419-432 (1983). An organic vehiclesystem may consist of a solvent (such as butyl ether) (Bernazzani etal., J. Chem. Therm. 33:629-641 (2001)) and a solute (such as ethylcellulose) (Stanton, Int. J. Hybrid Microelec. 6:419-432 (1983)), whichserves to enhance the dispersion and suspension. Kumar, Active & PassiveElec. Comp. 25:169-179(2002). Ethyl cellulose has the further advantageof its slight conductivity. Khare et al., Polym. Int. 42:138-142 (1997);Khare et al., Polym. Int. 49:719-727 (2000).

Another organic vehicle is polyethylene glycol (PEG), a polymer of lowmolecular weight (400 amu), which is different from silicone in its lowviscosity. By using PEG in conjunction with boron nitride particles as athermal paste between copper disks, a thermal contact conductance of1.9×10 ⁵ W/m².° C. has been attained. This value is higher than thatobtained by using a thermal paste involving silicone and boron nitridepowder (1.1×10⁵ W/m².° C.), but is lower than that obtained by usingsolder, applied in the molten state (2.1×10⁵/m².° C.). Xu et al., J.Electron. Pkg. 124:188-191 (2002). In fact, all thermal pastespreviously reported are inferior to solder in providing high thermalcontact conductance.

Carbon black is a very fine particulate form of elemental carbon,consisting of typically spherical particles, which in turn come togetherto form porous agglomerates. Carbon black is produced either byincomplete combustion or thermal decomposition of a hydrocarbonfeedstock. Types of carbon black include soot, lamp black (typicalparticle size 50-100 nm), channel black (typical particle size 10-30nm), furnace black (typical particle size 10-80 nm), thermal black(typical particle size 150-500 nm), and acetylene black (typicalparticle size 35-70 nm).

Carbon black is used as a low-cost electrically conductive filler inpolymers. Nakamura et al., NEC Res. & Dev. 83:121-127 (1986); Saad etal., J. Appl. Polym. Sci. 73:2657:2670 (1999). Due to its relatively lowthermal conductivity, however, carbon black has not been reported as afiller for thermally conductive pastes. Most commonly, it is used as areinforcement in rubber. Takirio et al., Tire Sci. & Tech. 26:241-257(1998); Haws et al., Rub. Div. Symp., ACS, Akron, Ohio 1:257-281 (1982);Hess et al., Rub. Chem. & Tech. 56:390-417 (1983); Kundu et al., J.Appl. Polym. Sci. 84:256-260 (2002); Ramesan et al., Plas. Rub. & Comp.30:355-362 (2001); Sridhar et al., J. Appl. Polym. Sci. 82:997-1005(2001).

In addition, carbon black is used in electrochemical electrodes (Takeiet al., J Power Sources 55:191-195 (1995); Van Deraerschot et al.,Electrochem. Soc. Ext. Abst., Electrochem. Soc., Pennington, N.J. 84:139(1984)), inks (Erhan et al., J. Am. Oil Chem. Soc. 68:635-638 (1991);Bratkowska et al., Przemysl Chemiczny 66:393-395 (1987); Bratkowska etal., Przemysl Chemiczny 65:363-365(1986)), lubricants (Chinas-Castilloet al., Tribology Trans. 43:387-394 (2000); Shiao et al., J. Appl.Polym. Sci. 80:1514-1519 (2001); Kozlovtsev et al., Glass & Ceramics(English Translation of Steklo I Keramika) 154-157; Bakaleinikov et al.,Chem. & Tech. Fuels & Oils 18:108-111 (1982)), fuels (Srivastava et al.,Fuel 73:1911-1917 (1984); Steinberg, Preprints: Div. Pet. Chem., ACS32:565-571 (1987); Smith, Automotive Eng. (London) 7:23-24, 27 (1982)),and pigments (Ueki et al., Ann. Conf Elec. Ins. & Dielec. Phen., Ann.Rpt., IEEE, Piscataway, N.J. 1: 170-176 (1997)).

The present invention is directed to overcoming these and otherdeficiencies in the art.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to, a thermally conductivepaste formed from porous agglomerates of carbon particles dispersed in apaste-forming vehicle.

Another aspect of the present invention is directed to a thermallyconductive interface material in the form of a paste formed from porousagglomerates of carbon particles dispersed in a paste-forming vehicle.

A further aspect of the present invention is an apparatus that includesa heat source, a heat sink proximate the heat source, and a layer of athermally conductive paste made in accordance with the present inventiondisposed between and in contact with the heat source and the heat sink.

Yet another aspect of the present invention is a method of removing heatfrom a heat source that involves providing a heat sink proximate theheat source and disposing a layer of a thermally conductive paste madein accordance with the present invention between and in contact with theheat source and the heat sink.

The thermally conductive paste of the present invention is highlyconformable and spreadable and is particularly useful as a thermallyconductive interface material. By using porous agglomerates of carbonparticles as the thermally conductive ingredient, thermal pastes thatare superior to solder in providing high thermal contact conductancehave been attained. Thermally conductive interface materials prepared inaccordance with the present invention can provide thermal contactconductance between copper disks of 3×10⁵ watts/meter².° C. (W/m².° C.),as compared to 2×10⁵ W/m².° C. for solder. Moreover, the pastes are easyto use and apply, unlike solder, which requires the application of heatduring use.

Thermally conductive pastes of the present invention have manyapplications. Pastes prepared in accordance with the present inventionmay be used, for example, for microelectronic cooling, for heat pipesfor tapping geothermal energy (Lockett, H. & V. Eng. 59:7-8; Lockett,Proc. Eur. Cong., 1:285-289 (1984)) and for thermal fluid heaters forproviding indirect process heat (Dawes et al., Inst. Energy, London, UKPap. KN/III/2 1:8 pp (1984)). Pastes prepared in accordance with thepresent invention may also be used, for example, for the cooling ofmachinery, boilers, cutting tools, oil drilling equipment components,spacecraft components and building components. Other applications may bein connection with foods, wound healing, therapeutics, etc.

Thermally conductive pastes of the present invention may also be used toimprove the thermal contact between a cold source and an objectproximate the cold source, for the purpose of cooling the object orother objects connected to the object. The pastes, may for example, beapplied to improve the thermal contact between a fluid-cooled object(the cold source) and a cold plate or a cold finger, for the purpose ofcooling an object connected to the cold plate or cold finger.

Another aspect of the present invention is therefore, an apparatus thatincludes a cold source, an object proximate the cold source, and a layerof a thermally conductive paste made in accordance with the presentinvention disposed between and in contact with the cold source and theobject.

Yet another aspect of the present invention is a method of improving thethermal contact between a first object and a second object proximate thefirst object, that involves disposing a layer of a thermally conductivepaste made in accordance with the present invention between and incontact with the first object and the second object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, perspective view of a layer of a thermallyconductive paste made in accordance with the present invention disposedbetween an integrated circuit chip and a heat sink.

FIG. 2 is a schematic representation of thermal contact conductancemeasurement in accordance with the present invention.

FIG. 3 is a graphic representation of thermogravimetric results obtainedfor (a) PEG by itself; and (b) PEG with 3 vol. % ethyl cellulose.

FIG. 4 is a graphic representation of thermogravimetric results obtainedfor (a) butyl ether by itself; and (b) butyl ether with 40 vol. % ethylcellulose.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a thermally conductive paste formedfrom porous agglomerates of carbon particles dispersed in apaste-forming vehicle. The paste is particularly useful as a thermallyconductive interface material for improving thermal contacts, such as,for example, between a heat source and a heat sink or between a coldsource and an object. Thus, the present invention further relates to athermally conductive interface material in the form of a paste formedfrom porous agglomerates of carbon particles dispersed in apaste-forming vehicle.

The porous agglomerates of carbon particles will preferably be a carbonblack. Types of carbon black include soot, lamp black (typical particlesize 50-100 nm), channel black (typical particle size 10-30 nm), furnaceblack (typical particle size 10-80 nm), thermal black (typical particlesize 150-500 nm), and acetylene black (typical particle size 35-70=m),for example.

In one aspect, the paste-forming vehicle may be a paste-forming vehiclesystem. Typically, a paste-forming vehicle system will include apaste-forming solvent and a solute. The system may, for example, beorganic-based or inorganic based. Examples of solvents that may be usedin a paste-forming vehicle system can include, but are not limited to,silicates (such as, for example, sodium silicate), glycol ethers (suchas, for example, di(ethylene glycol) butyl ether (“BE”)),methoxypolyethylene glycol (“MPEG”), ethylene glycol, propylene glycol,ethylene oxide, propylene oxide, polyethylene glycol (“PEG”), PEGmodified with various types of functional groups (such as, for example,—H, —CH₃, etc.) at the ends of the macromolecular chain, oil, water,alcohols (such as, for example, 2-ethylhexanol, 2-ethylhexoic acid,2-methyl butanol, propanol, ethanol, diacetone alcohol, isobutanol,isopropanol, n-butanol, n-pentanol, n-propanol, etc.), diethyl sulfate,diisobutyl carbinol, diisobutyl ketone, hexylene glycol, isobutylacetate, isophorone, isopropyl acetate, methyl isobutyl carbinol, ketone(such as, for example, methyl isobutyl ketone), n-butyl acetate,n-propyl acetate, primary amyl acetate mixed isomers, primary amylalcohol mixed isomers, n-propyl propionate, n-butyl propionate, n-pentylpropionate, methylene chloride, perchloroethylene, trichloroethylene,xylene, acetone, ethyl acetate, and chemically related substances.

Examples of solvents that may be used in a paste-forming vehicle systemcan include, but are not limited to, cellulosic resin, thermoplasticresin, glycidyl methacrylate, hydroxy(meth)acrylate monomers,epsilon-caprolactone monomer, hydroxypropyl acrylate, hydroxyethylacrylate, ethylene acrylic acid, divinylbenzene, styrene-butadienelatexes, acrylic latexes, vinyl acrylic latexes, styrene acryliclatexes, vinyl versatate latexes, vinyl chloride, vinylbenzyl chloride,chloromethylstyrene, vinyl acetate copolymers, epoxy resins, epoxyacrylate, aminoethylethanolamine, glycol ethers, propylene glycols,ethylene glycols, polyols (e.g., aliphatic polyester polyols), ethyleneacrylic acid resins, methylcellulose, ethyl cellulose, hydroxyethylcellulose, polyvinyl alcohol, starch, and chemically related substances.The choice of solute will typically depend on the choice of solvent. Atleast the last four solutes, for example, are soluble in water.

An organic-based paste-forming vehicle system may include, for example,a solvent such as polyethylene glycol or di(ethylene glycol) butylether, and a solute, such as, for example, ethyl cellulose.

In some instances, such as with water-based and alcohol-based pastes,the solvent evaporates, allowing the conductive units (i.e., carbonblack agglomerates) to be in intimate contact after the paste has dried.In contrast, oil-based and some polymer-based pastes do not dry. Etheritself is volatile, but the dissolution of a solute such as ethylcellulose in it results in a paste-forming vehicle that is not volatile,such that the resulting paste does not dry, as in the case of certainpolymer-based pastes.

In one aspect, a thermally conductive paste prepared in accordance withthe present invention may, for example, incorporate a paste-formingvehicle system including PEG as a solvent and ethyl cellulose as asolute. In this aspect, the amount of ethyl cellulose present in thepaste will preferably be within the range of about 3 to about 5 volumepercent (vol. %). More preferably, the amount of ethyl cellulose presentin the paste will be about 3 vol. %. Furthermore, the amount of carbonparticles dispersed in the paste will preferably be less than about 2.0vol. %, and more preferably, less than about 1.5 vol. %.

A particularly useful thermally conductive paste of the presentinvention includes porous agglomerates of carbon particles dispersed ina paste-forming vehicle system including ethyl cellulose in PEG, wherethe amount of carbon particles dispersed in the paste is about 1.25 vol.% and the amount of ethyl cellulose present in the paste is about 3 vol.%.

In another aspect, a thermally conductive paste prepared in accordancewith the present invention may, for example, incorporate a paste-formingvehicle system including di(ethylene glycol) butyl ether as a solventand ethyl cellulose as a solute. In this aspect, the amount of ethylcellulose present in the paste will preferably be about 40 vol. %. Inaddition, the amount of carbon particles dispersed in the paste willpreferably be less than about 0.40 vol. %, and more preferably, about0.20 vol. %.

Another particularly useful thermally conductive paste of the presentinvention includes porous agglomerates of carbon particles dispersed ina paste-forming vehicle system including ethyl cellulose in di(ethyleneglycol) butyl ether, where the amount of carbon particles dispersed inthe paste is about 0.20 vol. % and the amount of ethyl cellulose presentin the paste is about 40 vol. %.

Due to its high conformability and spreadability, the thermallyconductive paste of the present invention is especially useful as athermally conductive interface material to assist in heat transferbetween a heat source and a heat sink, for example, between anintegrated circuit chip and a heat sink for dissipation of heat from anelectronic apparatus.

Particularly useful as a thermally conductive interface material is apaste of the present invention where the paste-forming vehicle is apaste-forming vehicle system including ethyl cellulose in PEG and theamount of carbon particles dispersed in the paste is about 1.25 vol. %and the amount of ethyl cellulose present in the paste is about 3 vol.%.

Also useful as a thermally conductive interface material is a paste ofthe present invention where the paste-forming vehicle is a paste-formingvehicle system including ethyl cellulose in di(ethylene glycol) butylether, where the amount of carbon particles dispersed in the paste isabout 0.20 vol. % and the amount of ethyl cellulose present in the pasteis about 40 vol. %.

Referring now to FIG. 1, an electronic apparatus 10 made in accordancewith the present invention includes a heat source 12, such as, anintegrated circuit chip, and a heat sink 14. A layer 16 of a thermallyconductive paste made in accordance with the present invention isdisposed as an interface material between and in contact with the heatsource 12 and the heat sink 14. While the layer 16 of thermallyconductive paste can be formed in a variety of shapes and sizes to fillparticular needs, it is preferred in this context that it substantiallycover the entire surfaces of the heat source/heat sink interface.

As illustrated, the heat source 12 is mounted to a circuit board 18. Theheat source 12 is operably connected to an electrical source (not shown)and operates conventionally. As heat is generated by the heat source 12,the heat is conducted from a heat source outer surface 13 across thelayer of thermally conductive paste of the present invention to a heatsink inner surface 15. The heat is thereafter conventionally dissipatedto the atmosphere through the heat sink 14, as known in the art.Additionally, because the layer 16 of thermally conductive pastesubstantially covers both the heat source outer surface 13 and the heatsink inner surface 15, thermal contact resistance is minimized.

Accordingly, the present invention is also directed to an electronicapparatus that includes a heat source, a heat sink, and a layer of athermally conductive paste of the present invention disposed between andin contact with the heat source and the heat sink.

The present invention is yet further directed to a method of removingheat from a heat source which involves providing a heat sink proximatethe heat source and disposing a layer of a thermally conductive paste ofthe present invention between and in contact with the heat source andthe heat sink. The method is useful, for example, in aiding in thedissipation of heat from a microelectronic device or apparatus.

The examples that follow are focused on the addition of variousthermally conductive fillers to organic vehicles for the purpose ofproviding a thermal paste which is conformable, spreadable andrelatively high in thermal conductivity. The fillers evaluated here arecarbons (such as, carbon black, 1 μm and 5 μm graphite particles, 0.1 μmdiameter discontinuous carbon filaments and 25 μm diamond particles) and1 μm and 3 μm nickel particles.

Of all these fillers, carbon black, which is porous, is the only typethat is itself spreadable (compressible). Galli, Plastics Compounding5:22-32 (1982), which is hereby incorporated by reference in itsentirety. The spreadability is believed to be the reason behind thesuperior performance of carbon black compared to all the other fillersinvestigated. Helsen et al., Colloid. & Polym. Sci. 264:619-622 (1986);Mewis et al., Colloids & Surfaces 22:271-289 (1987); Genz et al., J.Colloid & Interface Sci. 165:212-220 (1994), which are herebyincorporated by reference in their entirety.

Furthermore, the porosity of a carbon black particle allows penetrationof the vehicle into a carbon black particle, thereby enabling theresulting paste to have high fluidity, as previously shown for the caseof oil as the vehicle. Ishii et al., Carbon 39:2384-2386 (2001); Trappeet al., Phys. Rev. Lett. 85:449-452 (2000); Kratohvil et al., Colloids &Surfaces 5:179-186 (1982); Fitzgerald et al., Rubber Chem. & Tech.55:1569-1577 (1982); Amari, Progress in Organic Coatings 31:11-19(1997), which are hereby incorporated by reference in their entirety.

The examples further provide a comparative study of various organicvehicles and vehicle systems, such as, PEG with 0-15 vol. % dissolvedethyl cellulose and di(ethylene glycol) butyl ether with 0-40 vol. %dissolved ethyl cellulose. The comparative evaluation pertains both tothe effectiveness of the thermal paste and the temperature resistance,as both attributes are relevant to the thermal paste application.

EXAMPLES Example 1 Preparation of Paste Formulations

The polyethylene glycol, HO(CH₂CH₂O)₁₁H, (“PEG”) used as an organicvehicle was PEG 400 (EM Science, Gibbstown, N.J.). It had a molecularweight of 400 amu. It was a liquid at room temperature and optionallycontained ethyl cellulose (Sigma Chemical Co., St. Louis, Mo.) at either3 or 5 vol. %. The ethyl cellulose was a white powder that was dissolvedin the vehicle. It served to improve the dispersion and suspension ofthe solids in the pastes.

The other organic vehicle used was di(ethylene glycol) butyl ether(Aldrich Chemical Co., Inc., Milwaukee, Wis.). It optionally containedethyl cellulose (Sigma Chemical Co., St. Louis, Mo.) at 10, 20, 30 or 40vol. %.

The carbon black used was a type for electrical conductivity and easydispersion (Vulcan XC72R GP-3820; Cabot Corp., Billerica, Mass.). Itconsisted of porous agglomerates of carbon particles of particle size 30nm, density 1.7-1.9 g/cm³, nitrogen specific surface area 254 m²/g andmaximum ash content 0.2%. The carbon black powder was mixed with avehicle by hand, stirring to form a uniform paste.

Other thermally conductive solids, all used at 0.27 vol. % for the sakeof comparison, were graphite particles (Asbury Graphite Mills, Inc.,Asbury, N.J., (i) artificial graphite, Grade Ultra Fine 440, 99.4%typical carbon content, 1 μm typical size, and (ii) natural crystallineflake, Grade Micro 850, 98.5% minimum carbon content, 5 μm typicalsize), carbon filaments (Applied Sciences Inc., Cedarville, Ohio, 0.1 μmdiameter, >100 μm length, with intertwined morphology and fishbonetexture),1 μm nickel particles (INCO, Inc., Missassauga, Ontario,Calif., Type 210), 3 μm nickel particles (Novamet Specialty ProductsCorp., Wyckoff, N.J., Type 525, 15 to 20 μm length), and 25 μm diamondparticles (Warren Superabrasives, Olyphant, Pa., Type MB).

The pastes were prepared by first dissolving ethyl cellulose (ifapplicable) to the vehicle (either PEG or butyl ether). The dissolutionwas performed at room temperature for butyl ether, but at about 60° C.(with heat provided by a hot plate) for PEG. The heating for the case ofPEG was to hasten the dissolution. After this, the thermally conductivesolid ingredient was added. Mixing was conducted by using a ball milland stainless steel balls. After mixing, the paste was placed in avacuum chamber (which involved a mechanical vacuum pump) for the purposeof air bubble removal.

Example 2 Thermal Contact Conductance Measurement

As generally depicted in FIG. 2, a layer of a thermally conductive paste16 (or solder) was sandwiched between the flat surfaces of two copperdisks 20,22 (both surfaces of each disk having been mechanicallypolished by using 0.05 μm alumina particles), which had diameter 12.6 mmand thickness of 1.16 mm for one disk and 1.10 mm for the other disk.The thermal contact conductance between two copper disks with andwithout a layer of thermally conductive paste 16 (or solder) wasmeasured using the transient laser flash method. Xu et al., J. Electron.Pkg. 124:188-191 (2002); Xu et al., J Electron. Pkg. 122:128-131 (2000);Parker et al., J. Appl. Phys. 32:1679-1683 (1961); Inoue et al., YosetsuGakkai Ronbunshu/Quarterly J Jap. Welding Soc., 6:130-134 (1988), whichare hereby incorporated by reference in their entirety.

The pressure on the sandwich was controlled at 0.46, 0.69 and 0.92 MPa(depicted in FIG. 2 as arrow A). This is because the pressure affectsthe thermal contact conductance, even for a material that is notresilient. Xu et al., J. Electron. Pkg. 122:128-131 (2000), which ishereby incorporated by reference in its entirety. The thickness of thethermally conductive paste was 25 μm or less. The uniform distributionof the paste in the plane of the sandwich was made possible by thefluidity of the paste and the use of pressure. The thermally conductivepaste thickness was obtained by subtracting the thicknesses of the twocopper disks from the thickness of the sandwich, such that allthicknesses were measured using a micrometer. The thermally conductivepaste thickness for all cases was the same before and after theconductance measurement.

For the sake of comparison, solder (applied in the molten state) wasalso used as a thermal interface material (i.e., substituted for thelayer of thermally conductive paste 16). The solder wastin-lead-antimony (63 Sn-36.65 Pb-0.35 Sb), with activated Rosin fluxcore (Solder Type 361A-20R by Measurements Group, Inc., Raleigh, N.C.).Molten solder at a temperature of 187° C., as measured by using a Type-Tthermocouple, was sandwiched between copper disks that had beenpreheated to this temperature also. This temperature was above theliquidus temperature of 183° C. The heat was provided by a hot plate.The copper-solder-copper sandwich was allowed to cool on the hot platewith the power off under slight pressure. The thickness of the solderwas 25 μm or less.

The finite element program ABAQUS® (Abaqus, Inc., Pawtucket, R1) wasused to calculate the thermal contact conductance through temperaturevs. time curves, which were experimentally obtained. Xu et al., JElectron. Pkg. 122:128-131 (2000), which is hereby incorporated byreference in its entirety. The calculation assumed no thickness and noheat capacity for the interface between the two copper disks. Inaddition, it assumed no heat transfer between specimen and environmentexcept for the absorption of laser energy by the specimen. Moreover, itassumed that the laser energy was uniformly absorbed on the surface ofthe specimen, that the heat flow was one-dimensional, and that thethermal contact conductance between the two copper disks was uniform.The validity of these assumptions is supported by the calibration resultand error analysis given below.

Referring again to FIG. 2, a Coherent General Everpulse Model 11 Ndglass laser (Coherent, Inc., Santa Clara Calif.) (depicted generally inFIG. 2 as 24) with a pulse duration of 0.4 ms, a wavelength of 1.06 μmand a pulse energy up to 15 J was used for impulse heating. The laserpower was adjusted to allow the temperature rise of the specimen to bebetween 0.5 and 1.0° C. The upper surface of disk #1, 20, on which thelaser beam (depicted as arrow B) would directly hit had been coated bycarbon in order to increase the extent of laser energy absorptionrelative to the extent of reflection. A first E-type thermocouple (notshown) was attached to the back surface of disk #2, 22, for monitoringthe temperature rise. A second thermocouple of the same type (not shown)was put ˜30 cm above the specimen holder to detect the initial time thatthe laser beam (arrow B) came out.

A National Instruments DAQPad-MIO 16XE-50 data acquisition board(National Instruments, Austin, Tex.) with a data acquisition rate up to20,000 data points per second at 16 bites resolution, along with NI-DAQinterface software coded in Visual Basic® (Microsoft® Corp., Redmond,Wash.) was used to monitor the response of both thermocouplessimultaneously. A plexiglass sample holder 26, bolt 28 and rubberinsulator 30, were used to facilitate pressure application. A SensotecModel 13 (Columbus, Ohio) load cell 32 was used for pressuremeasurement. Calibration using a standard graphite specimen wasperformed before testing each specimen in order to ensure measurementaccuracy. The data acquisition rate used for each test was adjusted sothat there were at least 100 temperature data points during thetemperature rise.

The experimental error in transient thermal contact conductancemeasurement consists of random error due to experimental data scatter,and systematic error mainly due to the lag of the thermocouple responseand partly due to the method used to calculate the conductance from thetemperature data. The higher the thermal contact conductance, thegreater is the error. The thermal diffusivity of a standard NBS 8426graphite disk (thickness =2.62 mm), which had a similar transienttemperature rise time as the copper sandwich with the highest thermalcontact conductance, was measured prior to testing each specimen inorder to determine the systematic error, if any. The random error shownby the ± value was determined by measurement of five specimens.

Example 3 Viscosity Testing

The viscosity of the formulations was measured by using a viscometer(Brookfield Engineering Laboratories, Inc., Middleboro, Mass., Model LVTDial-Reading Viscometer, with Model SSA-18/13R Small Sample Adaptor).

Example 4 Thermal Stability Testing

The thermal stability of the formulations was tested bythermogravimetry, as conducted by heating in air from room temperatureto 150° C. at a rate of 2° C./min. A Perkin Elmer(Newark, Conn.) TGA7instrument was used.

Example 5 Evaluation of Thermal Contact Conductance

Table 1 gives thermal contact conductance for various thermal pastes(containing 0 to 3.20 vol. % carbon black) between copper disks atvarious contact pressures. The paste was below 25 μm in thickness. Asseen in Table 1, without carbon black, the optimum ethyl cellulosecontent for attaining high thermal contact conductance using PEG as thevehicle base was 5 vol. %. The conductance was less when the ethylcellulose content was below or above this value. This is attributed tothe increase in both conductivity and viscosity provided by the ethylcellulose. The conductivity helped the contact conductance, thus causingthe conductance to increase as the cellulose content increased from 0 to5 vol. %. On the other hand, the viscosity was detrimental to thecontact conductance, thus causing the conductance to decrease as thecellulose content increased from 5 to 15 vol. %.

The addition of carbon black to PEG containing 3 vol. % ethyl celluloseincreased the contact conductance, provided that the carbon blackcontent was 1.25 vol. % or below (Table 1). Exceeding this contentcaused the contact conductance to drop, as shown for a carbon blackcontent of 1.5 vol. %, which gave even lower conductance than the casewithout carbon black. In the case of PEG containing 5 vol. % ethylcellulose, the conductance was diminished greatly by the carbon blackaddition (even just 0.5 vol. % carbon black). This is attributed to theresulting high viscosity when ethyl cellulose was present at 5 vol. %and the further increase in viscosity upon the carbon black addition.The viscosity results are described in Example 6. Among the PEG basedpastes, the highest contact conductance of 30×10⁴ W/m².° C. was attainedby using 3 vol. % ethyl cellulose and 1.25 vol. % carbon black.

Referring again to Table 1, it is seen that for butyl ether withoutcarbon black, the optimum ethyl cellulose content for attaining highcontact conductance was 30 vol. % when the contact pressure was 0.46 MPaand was 20 vol. % when the pressure was 0.69 or 0.92 MPa. Due to its lowviscosity, butyl ether dissolved ethyl cellulose powder more easily thandid PEG. However, the conductance attained by butyl ether, whether withor without ethyl cellulose, is lower than that attained by PEG, whetherwith or without carbon black.

The addition of carbon black to butyl ether had little effect on thecontact conductance, unless the ethyl cellulose content was high (40vol. %). In this case, the conductance increased with carbon blackcontent from 0 to 0.20 vol. % and decreased with carbon black contentfrom 0.20 to 0.53 vol. %. The optimum carbon black content 0.20 vol. %,at which conductance reached 28×10⁴ W/m².° C.

For pastes based on PEG (with 3 vol. % ethyl cellulose) and butyl ether(with 40 vol. % ethyl cellulose), the conductance was maximum at anintermediate content of carbon black. This trend had been previouslyreported for boron nitride particle pastes based on lithium doped PEG.Xu et al., J Electron. Pkg., 124: 188-191 (2002), which are herebyincorporated by reference in their entirety. It is attributed to therequired compromise between thermal conductivity and viscosity, both ofwhich increase with increasing solid content. The viscosity results aredescribed in Example 6.

The highest conductance attained by PEG-based and butyl ether-basedpastes is similar. However, the optimum carbon black content is muchhigher for the PEG-based paste and the optimum ethyl cellulose contentis much lower for the PEG-based paste. Due to the importance of a lowviscosity, the use of a high ethyl cellulose content requires that of alow carbon black content, and the use of a high carbon black contentrequires that of a low ethyl cellulose content. TABLE 1 ThermalInterface Material Vol. % Vol. % Conductance (10⁴ W/m² · ° C.) VehicleEC CB 0.46 MPa 0.69 MPa 0.92 MPa PEG 0 0 11.00 ± 0.30 — — PEG 3 0 12.02± 0.86 13.98 ± 1.06 15.57 ± 1.03 PEG 5 0 18.51 ± 0.83 18.92 ± 0.91 20.74± 1.52 PEG 7.5 0 17.61 ± 0.11 17.60 ± 0.75 19.21 ± 0.79 PEG 10 0 12.31 ±0.52 12.29 ± 0.61 14.69 ± 0.80 PEG 15 0  4.14 ± 0.25  4.41 ± 0.07  4.59± 0.33 PEG 3 0 12.02 ± 0.86 13.98 ± 1.06 15.57 ± 1.03 PEG 3 0.50 15.45 ±0.94 17.67 ± 1.09 19.10 ± 0.43 PEG 3 1.00 18.83 ± 1.08 19.41 ± 1.3822.81 ± 1.12 PEG 3 1.25 29.90 ± 0.79 28.98 ± 2.11 29.63 ± 1.92 PEG 31.50  9.92 ± 0.57 11.50 ± 0.90 12.29 ± 1.06 PEG 5 0 18.51 ± 0.83 18.92 ±0.91 20.74 ± 1.52 PEG 5 0.50  9.00 ± 0.14 13.16 ± 0.19 13.28 ± 0.07 PEG5 0.75 11.71 ± 0.44 12.90 ± 0.31 14.83 ± 0.63 PEG 5 1.00 10.61 ± 0.2011.45 ± 0.33 11.61 ± 0.50 BE 0 0  2.89 ± 0.10 —  3.86 ± 0.08 BE 10 0 3.65 ± 0.13  4.55 ± 0.21  5.68 ± 0.06 BE 20 0  3.70 ± 0.08  5.11 ± 0.05 6.40 ± 0.11 BE 30 0  4.60 ± 0.28  5.08 ± 0.15  5.54 ± 0.21 BE 40 0 3.67 ± 0.13  4.37 ± 0.12  4.61 ± 0.06 BE 0 0  2.89 ± 0.10 —  3.86 ±0.08 BE 0 1.34  2.14 ± 0.08 —  3.75 ± 0.06 BE 0 2.14  2.85 ± 0.04 — 3.08 ± 0.08 BE 0 2.67  1.64 ± 0.10 —  2.37 ± 0.0+ BE 0 3.20  1.62 ±0.07 —  2.32 ± 0.06 BE 10 0  3.65 ± 0.13  4.55 ± 0.21  5.68 ± 0.06 BE 100.53  1.10 ± 0.06  2.99 ± 0.06  4.42 ± 0.06 BE 10 1.34  4.53 ± 0.15 5.35 ± 0.19  5.43 ± 0.31 BE 10 2.14  3.75 ± 0.11  4.64 ± 0.22  4.75 ±0.17 BE 10 2.67  1.75 ± 0.05  2.75 ± 0.06  4.05 ± 0.18 BE 20 0  3.70 ±0.08  5.11 ± 0.05  6.40 ± 0.01 BE 20 0.53  4.02 ± 0.13  5.17 ± 0.09 5.47 ± 0.28 BE 20 1.34  4.13 ± 0.13  5.25 ± 0.16  5.52 ± 0.11 BE 202.14  5.00 ± 0.17  5.39 ± 0.13  5.64 ± 0.20 BE 20 2.67  1.08 ± 0.07 1.13 ± 0.03  1.45 ± 0.03 BE 30 0  4.60 ± 0.28  5.08 ± 0.15  5.54 ± 0.21BE 30 0.27  3.41 ± 0.14  3.94 ± 0.10  4.17 ± 0.05 BE 30 0.53  4.23 ±0.16  5.60 ± 0.22  6.62 ± 0.32 BE 30 1.07  1.65 ± 0.02  2.13 ± 0.05 2.88 ± 0.07 BE 40 0  3.67 ± 0.13  4.37 ± 0.12  4.61 ± 0.06 BE 40 0.1010.90 ± 1.10 16.19 ± 1.02 16.93 ± 0.12 BE 40 0.20 27.43 ± 2.75 28.41 ±2.12 28.03 ± 1.57 BE 40 0.27 18.94 ± 0.60 24.87 ± 1.00 25.74 ± 1.20 BE40 0.30 13.62 ± 1.32 17.05 ± 1.26 18.54 ± 1.53 BE 40 0.40  6.02 ± 0.58 7.68 ± 0.10  9.56 ± 0.62 BE 40 0.53  4.95 ± 0.15  5.58 ± 0.17  5.55 ±0.11EC = ethyl celluloseCB = carbon blackPEG = polyethylene glycolBE = di(ethylene glycol) butyl ether

Table 2 gives thermal contact conductance for thermal pastes in the formof di(ethylene glycol) butyl ether containing 40 vol. % ethyl celluloseand 0.27 vol. % thermally conductive solids, as tested between copperdisks at various contact pressures. The paste was below 25 μm inthickness.

Table 2 shows that carbon black is a much more effective conductiveadditive than graphite, nickel and diamond particles and carbonfilaments, for it provides a thermal paste that gives an exceptionallyhigh thermal contact conductance. The superiority of carbon black occursin spite of the relatively poor thermal conductivity of carbon black. Itis attributed to the conformability and spreadability of the paste, asenhanced by the compressibility of the carbon black agglomerates.

The compressibility of carbon black and the consequent electricalconnectivity attained upon squeezing, have been previously reported. Inparticular, as an electrically conductive additive to a non-conductiveMn0₂ particle cathode of an electrochemical cell, carbon black resultedin a lower resistivity than carbon filament without graphitization (sameas the carbon filament used in this work), due to the squeezing of thecarbon black between adjacent Mn0₂ particles. Frysz et al., J. PowerSources 58:41-54 (1996); Lu et al., Carbon 40:447-449 (2000), which arehereby incorporated by reference in their entirety. In contrast, theother conductive solids investigated are not compressible. Carbon blackis even superior to single-walled carbon nanotubes, pastes of which weretested using the methods and equipment described here. Xu et al., JElectron. Mater. (2004), which is hereby incorporated by reference inits entirety. For comparison, these results are shown in Table 2.Conformability and spreadability are more important than thermalconductivity in governing thermal paste performance.

The use of solder in place of a thermal paste gave a thermal contactconductance of (20.08±0.60)×10⁴ W/m².° C. (not shown in Tables). Thisvalue is consistent with that previously reported in the literature forthe same testing method and configuration. Xu et al., J. Electron. Pkg.124: 188-191 (2002), which is hereby incorporated by reference in itsentirety. Thus, the optimized carbon black pastes of the presentinvention are significantly superior to solder as thermally conductiveinterface materials.

The limited effectiveness of solder occurs in spite of the high thermalconductivity of solder. This is partly due to the reaction betweensolder and the copper disks. This reaction results in copper-tinintermetallic compounds at the solder-copper interface. Grivas et al., JElectron Mater. 15:355-359 (1986); Tu, Mater. Chem. Phys. 46:217-223(1996); Tsutsumi et al., Int. J. Hybrid Microelec. 7:38-43 (1984), whichare hereby incorporated by reference in their entirety. The compoundformation causes the solder to not wet the copper surface. Kim et al.,Mat. Res. Soc. 183-188 (1995), which is hereby incorporated by referencein its entirety. This leads to increased difficulty of the solder toconform to the surface topography of the copper. Conformability andspreadability are more important than thermal conductivity in governingthe performance of a thermal interface material.

The thermal contact conductance values reported herein for the pasteformulations (Tables 1 and 2) and solder as thermal interface materialswere all obtained using the same specimen configuration, testing methodand data analysis algorithm, and the values are reliable on a relativescale. However, the values deviate from the true values, due to the factthat the data analysis algorithm neglects the thickness of the thermalinterface material. Luo et al., Int. J. Microcircuits Electron. Pkg.24:141-147 (2001), which is hereby incorporated by reference in itsentirety. TABLE 2 THERMALLY CONDUCTIVE CONDUCTANCE (10⁴ W/m² · ° C.)SOLID 0.46 MPa 0.69 MPa 0.92 MPa Carbon Black 18.94 ± 0.60  24.87 ±1.00  25.74 ± 1.20  Graphite (5 μm) 3.03 ± 0.09 3.67 ± 0.08 4.02 ± 0.12Graphite (1 μm) 1.52 ± 0.03 1.77 ± 0.04 2.04 ± 0.05 Nickel (3 μm) 1.85 ±0.05 2.14 ± 0.02 2.84 ± 0.04 Nickel (1 μm) 0.91 ± 0.07 2.03 ± 0.10 2.66± 0.03 Diamond (25 μm) 1.15 ± 0.02 1.21 ± 0.09 1.54 ± 0.03 CarbonFilaments 1.09 ± 0.03 1.32 ± 0.02 1.51 ± 0.03 (0.1 μm diameter)Single-walled carbon 13.5 ± 0.2  13.8 ± 0.3  14.1 ± 0.4  nanotubes

Example 6 Evaluation of Viscosity

Table 3 shows the viscosity of selected pastes, as measured at variousappropriate shear rates. The addition of ethyl cellulose to either PEGor butyl ether monotonically increased the viscosity, as shown in theabsence of carbon black. PEG alone was higher in viscosity than butylether alone. However, PEG with the optimum ethyl cellulose content of 3vol. % was much lower in viscosity than butyl ether with the optimumethyl cellulose content of 40 vol. %. The addition of carbon blackmonotonically increased the viscosity, as shown for PEG containing 3vol. % ethyl cellulose and for butyl ether containing 40 vol. % ethylcellulose.

The PEG with 5 vol. % ethyl cellulose and the PEG-based paste containing3 vol. % ethyl cellulose and 1.25 vol. % carbon black were similar inviscosity. The latter gave a higher contact conductance than the former,due to a decrease in the ethyl cellulose content and an increase in thecarbon black content. Thus, adjustment of the contents of both ethylcellulose and carbon black is needed in order to attain an optimizedthermal paste formulation.

As shown in Table 1, the butyl ether-based paste with 40 vol. % ethylcellulose and 0.20 vol. % carbon black and the PEG-based paste with 3vol. % ethyl cellulose and 1.25 vol. % carbon black are the two thermalpastes of the present invention that gave the highest thermal contactconductance. As shown in Table 3, although the two pastes gave similarlyhigh values of the contact conductance, the butyl ether-based pasteexhibited a much higher viscosity than the PEG-based paste.

Table 3 also shows that the viscosity of the butyl ether-based pastewith 40 vol. % ethyl cellulose was lower when the paste contained 0.20vol. % graphite particles (1 or 5 μm) or carbon filaments than when itcontained 0.20 vol. % carbon black. However, it was noticed during pastemixing that the carbon black paste was much smoother than the graphiteparticle paste. The smoothness of the paste is apparently more importantthan the viscosity in governing thermal paste performance. Perhapssmoothness relates more closely to the conformability than a lowviscosity.

Referring again to Table 3, the viscosity of butyl ether-based pastewith 40 vol. % ethyl cellulose and 0.20 vol. % solid increased in theorder: 1 μm graphite particles, 5 μm graphite particles and carbonfilaments. This trend is consistent with the notion that a largerparticle size tends to result in a paste with a higher viscosity andthat filaments tend to result in a higher viscosity than particles.TABLE 3 Viscosity (cP) 0.79 2.0 2.6 4.0 6.6 7.9 16 40 Vehicle Vol. % ECVol. % CB (s⁻¹) (s⁻¹) (s⁻¹) (s⁻¹) (s⁻¹) (s⁻¹) (s⁻¹) (s⁻¹) PEG 0 0 — — —125 — 120 120 — PEG 3.0 0 — — — 160 — 145 140 — PEG 5.0 0 — — — 190 —190 195 — PEG 7.5 0 — — — 240 — 250 — — PEG 3.0 0.50 — — — 175 — 185 185— PEG 3.0 1.25 — — — 200 — 195 200 — PEG 3.0 1.50 — — — 210 — 210 215 —BE 0 0 — — — — — — — <10 BE 10 0 — — — — — —  25   20 BE 30 0 580 520 —540 — — — — BE 40 0 — — 4,000 — 3,900 — — — BE 40 0.10 — — 4,720 — 4,400— — — BE 40 0.20 — — 5,200 — 4,800 — — — BE 40 0.30 — — 5,600 — 5,200 —— — BE 40 0.20^(a) — — 4,000 — 3,900 — — — BE 40 0.20^(b) — — 4,800 —4,480 — — — BE 40 0.20^(c) — — 5,000 — 4,720 — — —EC = ethyl celluloseCB = carbon blackBE = di(ethylene glycol) butyl ether^(a)Graphite particles (1 μm)^(b)Graphite particles (5 μm)^(c)Carbon filaments

Although the viscosity is a useful suggestive indicator of thermal pasteperformance, it is not the same as the conformability, which is theattribute that really governs thermal paste performance. Since there isno standardized method of conformability measurement, this work resortedto viscosity measurement.

Example 7 Evaluation of Thermal Stability

FIGS. 3 and 4 respectively, show the thermogravimetric results for PEGand butyl ether (with and without ethyl cellulose in each case, butwithout carbon black, which does not affect the thermal stability of thedispersion). Without ethyl cellulose, PEG is much more stable thermallythan butyl ether. The dissolution of ethyl cellulose diminished thethermal stability of PEG, but increased that of butyl ether.

Table 4 shows a comparison of thermogravimetric results of these thermalpastes at three temperatures. At 50° C. and 75° C., butyl ethercontaining ethyl cellulose is more stable thermally than PEG containingethyl cellulose, but at 100° C., the reverse is true. Above about 100°C., the weight loss of butyl ether, whether with or without ethylcellulose, is extensive (more than 50% weight loss at 150° C.). However,the weight loss remains less than 9% even at 150° C. for PEG, whetherwith or without ethyl cellulose. Therefore, butyl ether-based pastes arenot suitable for use above 100° C., whereas PEG-based pastes aresuitable for use up to at least 150° C. TABLE 4 Thermal InterfaceMaterial Residual Weight (%) Vehicle Vol. % EC 50° C. 75° C. 100° C. PEG0 99.81 99.22 98.91 PEG 3 98.73 95.47 93.36 BE 0 98.98 96.11 84.67 BE 4099.33 97.26 89.68EC = ethyl cellulosePEG = polyethylene glycolBE = di(ethylene glycol) butyl ether

The use of a PEG-based paste (containing 3 vol. % dissolved ethylcellulose and dispersed carbon black in the optimum amount of 1.25 vol.%) as a thermal interface material between copper disks results in athermal contact conductance of 30×10⁴ W/m².° C., compared to a value of20×10⁴ W/m².° C. for tin/lead eutectic solder applied in the moltenstate. Almost as effective as the PEG-based paste is a butyl ether-basedpaste (containing 40 vol. % dissolved ethyl cellulose and dispersedcarbon black in the optimum amount of 0.20 vol. %), which gives athermal contact conductance of 28×10⁴ W/m².° C. The PEG-based paste issuperior to the butyl ether-based paste in the thermal stability at 100°C. and above, though the reverse is true at 75° C. and below. Thesuperiority of the formulations of the present invention to solder asthermal interface materials may presumably be due to the reactionbetween solder and copper and the consequent poor conformability ofmolten solder with copper.

The use of PEG by itself gives a thermal contact conductance of 11×10⁴W/m².° C. The dissolution of ethyl cellulose at an optimum concentrationof 5 vol. % increases the conductance to 19×10⁴ W/m².° C. The use ofbutyl ether by itself gives a contact conductance of 3×10⁴ W/m².° C. Thedissolution of ethyl cellulose at the optimum concentration of 20 to 30vol. % gives a conductance ranging from 4×10⁴ to 6×10⁴ W/m².° C.

The addition of carbon black to PEG helps the conductance when the ethylcellulose is at 3 vol. % and the carbon black content is at 1.25 vol. %or below. The addition of carbon black to PEG degrades the conductancewhen the ethyl cellulose is at 3 vol. % and carbon black is at 1.5 vol.%, or when the ethyl cellulose is at 5 vol. %. These effects arepresumably due to the importance of conformability and spreadability tothe thermal paste performance. Both carbon black and ethyl cellulosecause the viscosity of the paste to increase, so excessive amounts ofthese ingredients degrade the conductance.

The optimum carbon black content is higher for PEG than butyl etherbased, whereas the optimum ethyl cellulose content is higher for butylether than PEG. In spite of the difference in carbon black content, thethermal contact conductance is similar between the optimized PEG-basedpaste and the optimized butyl ether-based paste. Since carbon black isthe ingredient in the paste that is most conductive thermally, thisimplies that the conformability and spreadability are more importantthan the thermal conductivity in governing thermal paste performance.

Moreover, in spite of its own relatively low thermal conductivity,carbon black is much more effective than graphite, nickel and diamondparticles and carbon filaments, all evaluated at the same volumefraction, for providing thermal pastes. This is attributed to thecompressibility of a carbon black agglomerate and the consequentconformability and spreadability of the paste.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various additions, substitutions, modifications and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. A thermally conductive paste comprising porous agglomerates of carbonparticles dispersed in a paste-forming vehicle.
 2. The paste as claimedin claim 1, wherein the porous agglomerates of carbon particles comprisea carbon black.
 3. The paste as claimed in claim 1, wherein thepaste-forming vehicle is a paste-forming vehicle system comprising asolvent selected from the group consisting of silicates, glycol ethers,methoxypolyethylene glycol (“MPEG”), ethylene glycol, propylene glycol,ethylene oxide, propylene oxide, polyethylene glycol (“PEG”), PEGmodified with various types of functional groups at the ends of amacromolecular chain, oil, water, alcohols, diethyl sulfate, diisobutylcarbinol, diisobutyl ketone, hexylene glycol, isobutyl acetate,isophorone, isopropyl acetate, methyl isobutyl carbinol, ketone, n-butylacetate, n-propyl acetate, primary amyl acetate mixed isomers, primaryamyl alcohol mixed isomers, n-propyl propionate, n-butyl propionate,n-pentyl propionate, methylene chloride, perchloroethylene,trichloroethylene, xylene, acetone, ethyl acetate, and chemicallyrelated substances.
 4. The paste as claimed in claim 1, wherein thesolvent is an organic solvent.
 5. The paste as claimed in claim 4,wherein the solvent is polyethylene glycol.
 6. The paste as claimed inclaim 1, wherein the paste-forming vehicle is a paste-forming vehiclesystem comprising a solute selected from the group consisting ofcellulosic resin, thermoplastic resin, glycidyl methacrylate,hydroxy(meth)acrylate monomers, epsilon-caprolactone monomer,hydroxypropyl acrylate, hydroxyethyl acrylate, ethylene acrylic acid,divinylbenzene, styrene-butadiene latexes, acrylic latexes, vinylacrylic latexes, styrene acrylic latexes, vinyl versatate latexes, vinylchloride, vinylbenzyl chloride, chloromethylstyrene, vinyl acetatecopolymers, epoxy resins, epoxy acrylate, aminoethylethanolamine, glycolethers, propylene glycols, ethylene glycols, polyols, ethylene acrylicacid resins, methylcellulose, ethyl cellulose, hydroxyethyl cellulose,polyvinyl alcohol, starch, and chemically related substances.
 7. Thepaste as claimed in claim 5, wherein the paste-forming vehicle systemcomprises ethyl cellulose as a solute.
 8. The paste as claimed in claim7, wherein the amount of ethyl cellulose present in the paste is in therange of about 3 to about 5 vol. %.
 9. The paste as claimed in claim 8,wherein the amount of ethyl cellulose present in the paste is about 3vol. %.
 10. The paste as claimed in claim 7, wherein the amount ofcarbon particles dispersed in the paste is less than about 2 vol. %. 11.The paste as claimed in claim 10, wherein the amount of carbon particlesdispersed in the paste is less than about 1.50 vol. %.
 12. The paste asclaimed in claim 9, wherein the amount of carbon particles dispersed inthe paste is less than about 1.50 vol. %.
 13. The paste as claimed inclaim 12, wherein the amount of carbon particles dispersed in the pasteis about 1.25 vol. %.
 14. The paste as claimed in claim 13, wherein theporous agglomerates of carbon particles comprise a carbon black.
 15. Thepaste as claimed in claim 4, wherein the solvent is di(ethylene glycol)butyl ether.
 16. The paste as claimed in claim 15, wherein thepaste-forming vehicle system comprises ethyl cellulose as a solute. 17.The paste as claimed in claim 16, wherein the amount of ethyl cellulosepresent in the paste is about 40 vol. %.
 18. The paste as claimed inclaim 16, wherein the amount of carbon particles dispersed in the pasteis less than about 0.40 vol. %.
 19. The paste as claimed in claim 17,wherein the amount of carbon particles dispersed in the paste is lessthan about 0.40 vol. %.
 20. The paste as claimed in claim 19, whereinthe amount of carbon particles dispersed in the paste is about 0.20 vol.%.
 21. The paste as claimed in claim 20, wherein the porous agglomeratesof carbon particles comprise a carbon black.
 22. A thermally conductiveinterface material comprising the paste as claimed in claim
 1. 23. Athermally conductive interface material comprising the paste as claimedin claim
 14. 24. A thermally conductive interface material comprisingthe paste as claimed in claim
 21. 25. An apparatus comprising: a heatsource; a heat sink for removing heat from the heat source; and a layerof a thermally conductive paste disposed between and in contact with theheat source and the heat sink, the paste comprising porous agglomeratesof carbon particles dispersed in a paste-forming vehicle.
 26. Theapparatus as claimed in claim 25, wherein the porous agglomerates ofcarbon particles comprise a carbon black.
 27. The apparatus as claimedin claim 25, wherein the paste-forming vehicle is a paste-formingvehicle system comprising a solvent selected from the group consistingof silicates, glycol ethers, methoxypolyethylene glycol (“MPEG”),ethylene glycol, propylene glycol, ethylene oxide, propylene oxide,polyethylene glycol (“PEG”), PEG modified with various types offunctional groups at the ends of a macromolecular chain, oil, water,alcohols, diethyl sulfate, diisobutyl carbinol, diisobutyl ketone,hexylene glycol, isobutyl acetate, isophorone, isopropyl acetate, methylisobutyl carbinol, ketone, n-butyl acetate, n-propyl acetate, primaryamyl acetate mixed isomers, primary amyl alcohol mixed isomers, n-propylpropionate, n-butyl propionate, n-pentyl propionate, methylene chloride,perchloroethylene, trichloroethylene, xylene, acetone, ethyl acetate,and chemically related substances.
 28. The apparatus as claimed in claim25, wherein the solvent is an organic solvent.
 29. The apparatus asclaimed in claim 28, wherein the solvent is polyethylene glycol.
 30. Theapparatus as claimed in claim 25, wherein the paste-forming vehicle is apaste-forming vehicle system comprising a solute selected from the groupconsisting of cellulosic resin, thermoplastic resin, glycidylmethacrylate, hydroxy(meth)acrylate monomers, epsilon-caprolactonemonomer, hydroxypropyl acrylate, hydroxyethyl acrylate, ethylene acrylicacid, divinylbenzene, styrene-butadiene latexes, acrylic latexes, vinylacrylic latexes, styrene acrylic latexes, vinyl versatate latexes, vinylchloride, vinylbenzyl chloride, chloromethylstyrene, vinyl acetatecopolymers, epoxy resins, epoxy acrylate, aminoethylethanolamine, glycolethers, propylene glycols, ethylene glycols, polyols, ethylene acrylicacid resins, methylcellulose, ethyl cellulose, hydroxyethyl cellulose,polyvinyl alcohol, starch, and chemically related substances.
 31. Theapparatus as claimed in claim 29, wherein the paste-forming vehiclesystem comprises ethyl cellulose as a solute.
 32. The apparatus asclaimed in claim 31, wherein the amount of ethyl cellulose present inthe paste is in the range of about 3 to about 5 vol. %.
 33. Theapparatus as claimed in claim 32, wherein the amount of ethyl cellulosepresent in the paste is about 3 vol. %.
 34. The apparatus as claimed inclaim 31, wherein the amount of carbon particles dispersed in the pasteis less than about 2 vol. %.
 35. The apparatus as claimed in claim 34,wherein the amount of carbon particles dispersed in the paste is lessthan about 1.50 vol. %.
 36. The apparatus as claimed in claim 33,wherein the amount of carbon particles dispersed in the paste is lessthan about 1.50 vol. %.
 37. The apparatus as claimed in claim 36,wherein the amount of carbon particles dispersed in the paste is about1.25 vol. %.
 38. The apparatus as claimed in claim 37, wherein theporous agglomerates of carbon particles comprise a carbon black.
 39. Theapparatus as claimed in claim 28, wherein the solvent is di(ethyleneglycol).
 40. The apparatus as claimed in claim 39, wherein thepaste-forming vehicle system comprises ethyl cellulose as a solute. 41.The apparatus as claimed in claim 40, wherein the amount of ethylcellulose present in the paste is about 40 vol. %.
 42. The apparatus asclaimed in claim 40, wherein the amount of carbon particles dispersed inthe paste is less than about 0.40 vol. %.
 43. The apparatus as claimedin claim 41, wherein the amount of carbon particles dispersed in thepaste is less than about 0.40 vol. %.
 44. The apparatus as claimed inclaim 43, wherein the amount of carbon particles dispersed in the pasteis about 0.20 vol. %.
 45. The apparatus as claimed in claim 44, whereinthe porous agglomerates of carbon particles comprise a carbon black. 46.A method of removing heat from a heat source comprising: providing aheat sink proximate the heat source and disposing a layer of a thermallyconductive paste between and in contact with the heat source and theheat sink, the paste comprising porous agglomerates of carbon particlesdispersed in a paste-forming vehicle.
 47. The method as claimed in claim46, wherein the porous agglomerates of carbon particles comprise acarbon black.
 48. The method as claimed in claim 46, wherein thepaste-forming vehicle is a paste-forming vehicle system comprising asolvent selected from the group consisting of silicates, glycol ethers,methoxypolyethylene glycol (“MPEG”), ethylene glycol, propylene glycol,ethylene oxide, propylene oxide, polyethylene glycol (“PEG”), PEGmodified with various types of functional groups at the ends of amacromolecular chain, oil, water, alcohols, diethyl sulfate, diisobutylcarbinol, diisobutyl ketone, hexylene glycol, isobutyl acetate,isophorone, isopropyl acetate, methyl isobutyl carbinol, ketone, n-butylacetate, n-propyl acetate, primary amyl acetate mixed isomers, primaryamyl alcohol mixed isomers, n-propyl propionate, n-butyl propionate,n-pentyl propionate, methylene chloride, perchloroethylene,trichloroethylene, xylene, acetone, ethyl acetate, and chemicallyrelated substances.
 49. The method as claimed in claim 46, wherein thesolvent is an organic solvent.
 50. The method as claimed in claim 49,wherein the solvent is polyethylene glycol.
 51. The method as claimed inclaim 46, wherein the paste-forming vehicle is a paste-forming vehiclesystem comprising a solute selected from the group consisting ofcellulosic resin, thermoplastic resin, glycidyl methacrylate,hydroxy(meth)acrylate monomers, epsilon-caprolactone monomer,hydroxypropyl acrylate, hydroxyethyl acrylate, ethylene acrylic acid,divinylbenzene, styrene-butadiene latexes, acrylic latexes, vinylacrylic latexes, styrene acrylic latexes, vinyl versatate latexes, vinylchloride, vinylbenzyl chloride, chloromethylstyrene, vinyl acetatecopolymers, epoxy resins, epoxy acrylate, aminoethylethanolamine, glycolethers, propylene glycols, ethylene glycols, polyols, ethylene acrylicacid resins, methylcellulose, ethyl cellulose, hydroxyethyl cellulose,polyvinyl alcohol, starch, and chemically related substances.
 52. Themethod as claimed in claim 50, wherein the paste-forming vehicle systemcomprises ethyl cellulose as a solute.
 53. The method as claimed inclaim 52, wherein the amount of ethyl cellulose present in the paste isin the range of about 3 to about 5 vol. %.
 54. The method as claimed inclaim 53, wherein the amount of ethyl cellulose present in the paste isabout 3 vol. %.
 55. The method as claimed in claim 52, wherein theamount of carbon particles dispersed in the paste is less than about 2vol. %.
 56. The method as claimed in claim 55, wherein the amount ofcarbon particles dispersed in the paste is less than about 1.50 vol. %.57. The method as claimed in claim 54, wherein the amount of carbonparticles dispersed in the paste is less than about 1.50 vol. %.
 58. Themethod as claimed in claim 57, wherein the amount of carbon particlesdispersed in the paste is about 1.25 vol. %.
 59. The method as claimedin claim 58, wherein the porous agglomerates of carbon particlescomprise a carbon black.
 60. The method as claimed in claim 49, whereinthe solvent is di(ethylene glycol) butyl ether.
 61. The method asclaimed in claim 60, wherein the paste-forming vehicle system comprisesethyl cellulose as a solute.
 62. The method as claimed in claim 61,wherein the amount of ethyl cellulose present in the paste is about 40vol. %.
 63. The method as claimed in claim 61, wherein the amount ofcarbon particles dispersed in the paste is less than about 0.40 vol. %.64. The method as claimed in claim 62, wherein the amount of carbonparticles dispersed in the paste is less than about 0.40 vol. %.
 65. Themethod as claimed in claim 64, wherein the amount of carbon particlesdispersed in the paste is about 0.20 vol. %.
 66. The method as claimedin claim 65, wherein the porous agglomerates of carbon particlescomprise a carbon black.
 67. A method of improving a thermal contactbetween a first object and a second object proximate the first object,comprising disposing a layer of a thermally conductive paste between andin contact with the first object and the second object, the pastecomprising porous agglomerates of carbon particles dispersed in apaste-forming vehicle.
 68. The method as claimed in claim 67, whereinthe first object is a heat source.
 69. The method as claimed in claim67, wherein the first object is a cold source.
 70. The method as claimedin claim 67, wherein the porous agglomerates of carbon particlescomprise a carbon black.
 71. The method as claimed in claim 67, whereinthe paste-forming vehicle is a paste-forming vehicle system comprising asolvent selected from the group consisting of silicates, glycol ethers,methoxypolyethylene glycol (“MPEG”), ethylene glycol, propylene glycol,ethylene oxide, propylene oxide, polyethylene glycol (“PEG”), PEGmodified with various types of functional groups at the ends of amacromolecular chain, oil, water, alcohols, diethyl sulfate, diisobutylcarbinol, diisobutyl ketone, hexylene glycol, isobutyl acetate,isophorone, isopropyl acetate, methyl isobutyl carbinol, ketone, n-butylacetate, n-propyl acetate, primary amyl acetate mixed isomers, primaryamyl alcohol mixed isomers, n-propyl propionate, n-butyl propionate,n-pentyl propionate, methylene chloride, perchloroethylene,trichloroethylene, xylene, acetone, ethyl acetate, and chemicallyrelated substances.
 72. The method as claimed in claim 67, wherein thesolvent is an organic solvent.
 73. The method as claimed in claim 72,wherein the solvent is polyethylene glycol.
 74. The method as claimed inclaim 67, wherein the paste-forming vehicle is a paste-forming vehiclesystem comprising a solute selected from the group consisting ofcellulosic resin, thermoplastic resin, glycidyl methacrylate,hydroxy(meth)acrylate monomers, epsilon-caprolactone monomer,hydroxypropyl acrylate, hydroxyethyl acrylate, ethylene acrylic acid,divinylbenzene, styrene-butadiene latexes, acrylic latexes, vinylacrylic latexes, styrene acrylic latexes, vinyl versatate latexes, vinylchloride, vinylbenzyl chloride, chloromethylstyrene, vinyl acetatecopolymers, epoxy resins, epoxy acrylate, aminoethylethanolamine, glycolethers, propylene glycols, ethylene glycols, polyols, ethylene acrylicacid resins, methylcellulose, ethyl cellulose, hydroxyethyl cellulose,polyvinyl alcohol, starch, and chemically related substances.
 75. Themethod as claimed in claim 73, wherein the paste-forming vehicle systemcomprises ethyl cellulose as a solute.
 76. The method as claimed inclaim 75, wherein the amount of ethyl cellulose present in the paste isin the range of about 3 to about 5 vol. %.
 77. The method as claimed inclaim 76, wherein the amount of ethyl cellulose present in the paste isabout 3 vol. %.
 78. The method as claimed in claim 75, wherein theamount of carbon particles dispersed in the paste is less than about 2vol. %.
 79. The method as claimed in claim 78, wherein the amount ofcarbon particles dispersed in the paste is less than about 1.50 vol. %.80. The method as claimed in claim 77, wherein the amount of carbonparticles dispersed in the paste is less than about 1.50 vol. %.
 81. Themethod as claimed in claim 80, wherein the amount of carbon particlesdispersed in the paste is about 1.25 vol. %.
 82. The method as claimedin claim 81, wherein the porous agglomerates of carbon particlescomprise a carbon black.
 83. The method as claimed in claim 72, whereinthe solvent is di(ethylene glycol) butyl ether.
 84. The method asclaimed in claim 83, wherein the paste-forming vehicle system comprisesethyl cellulose as a solute.
 85. The method as claimed in claim 84,wherein the amount of ethyl cellulose present in the paste is about 40vol. %.
 86. The method as claimed in claim 84, wherein the amount ofcarbon particles dispersed in the paste is less than about 0.40 vol. %.87. The method as claimed in claim 85, wherein the amount of carbonparticles dispersed in the paste is less than about 0.40 vol. %.
 88. Themethod as claimed in claim 87, wherein the amount of carbon particlesdispersed in the paste is about 0.20 vol. %.
 89. The method as claimedin claim 88, wherein the porous agglomerates of carbon particlescomprise a carbon black.
 90. An apparatus comprising: a cold source; anobject proximate the cold source; and a layer of a thermally conductivepaste disposed between and in contact with the cold source and theobject, the paste comprising porous agglomerates of carbon particlesdispersed in a paste-forming vehicle.
 91. The apparatus as claimed inclaim 90, wherein the porous agglomerates of carbon particles comprise acarbon black.
 92. The apparatus as claimed in claim 90, wherein thepaste-forming vehicle is a paste-forming vehicle system comprising asolvent selected from the group consisting of silicates, glycol ethers,methoxypolyethylene glycol (“MPEG”), ethylene glycol, propylene glycol,ethylene oxide, propylene oxide, polyethylene glycol (“PEG”), PEGmodified with various types of functional groups at the ends of amacromolecular chain, oil, water, alcohols, diethyl sulfate, diisobutylcarbinol, diisobutyl ketone, hexylene glycol, isobutyl acetate,isophorone, isopropyl acetate, methyl isobutyl carbinol, ketone, n-butylacetate, n-propyl acetate, primary amyl acetate mixed isomers, primaryamyl alcohol mixed isomers, n-propyl propionate, n-butyl propionate,n-pentyl propionate, methylene chloride, perchloroethylene,trichloroethylene, xylene, acetone, ethyl acetate, and chemicallyrelated substances.
 93. The apparatus as claimed in claim 90, whereinthe solvent is an organic solvent.
 94. The apparatus as claimed in claim93, wherein the solvent is polyethylene glycol.
 95. The apparatus asclaimed in claim 90, wherein the paste-forming vehicle is apaste-forming vehicle system comprising a solute selected from the groupconsisting of cellulosic resin, thermoplastic resin, glycidylmethacrylate, hydroxy(meth)acrylate monomers, epsilon-caprolactonemonomer, hydroxypropyl acrylate, hydroxyethyl acrylate, ethylene acrylicacid, divinylbenzene, styrene-butadiene latexes, acrylic latexes, vinylacrylic latexes, styrene acrylic latexes, vinyl versatate latexes, vinylchloride, vinylbenzyl chloride, chloromethylstyrene, vinyl acetatecopolymers, epoxy resins, epoxy acrylate, aminoethylethanolamine, glycolethers, propylene glycols, ethylene glycols, polyols, ethylene acrylicacid resins, methylcellulose, ethyl cellulose, hydroxyethyl cellulose,polyvinyl alcohol, starch, and chemically related substances.
 96. Theapparatus as claimed in claim 94, wherein the paste-forming vehiclesystem comprises ethyl cellulose as a solute.
 97. The apparatus asclaimed in claim 96, wherein the amount of ethyl cellulose present inthe paste is in the range of about 3 to about 5 vol. %.
 98. Theapparatus as claimed in claim 97, wherein the amount of ethyl cellulosepresent in the paste is about 3 vol. %.
 99. The apparatus as claimed inclaim 96, wherein the amount of carbon particles dispersed in the pasteis less than about 2 vol. %.
 100. The apparatus as claimed in claim 99,wherein the amount of carbon particles dispersed in the paste is lessthan about 1.50 vol. %.
 101. The apparatus as claimed in claim 98,wherein the amount of carbon particles dispersed in the paste is lessthan about 1.50 vol. %.
 102. The apparatus as claimed in claim 101,wherein the amount of carbon particles dispersed in the paste is about1.25 vol. %.
 103. The apparatus as claimed in claim 102, wherein theporous agglomerates of carbon particles comprise a carbon black. 104.The apparatus as claimed in claim 93, wherein the solvent is di(ethyleneglycol).
 105. The apparatus as claimed in claim 104, wherein thepaste-forming vehicle system comprises ethyl cellulose as a solute. 106.The apparatus as claimed in claim 105, wherein the amount of ethylcellulose present in the paste is about 40 vol. %.
 107. The apparatus asclaimed in claim 105, wherein the amount of carbon particles dispersedin the paste is less than about 0.40 vol. %.
 108. The apparatus asclaimed in claim 106, wherein the amount of carbon particles dispersedin the paste is less than about 0.40 vol. %.
 109. The apparatus asclaimed in claim 108, wherein the amount of carbon particles dispersedin the paste is about 0.20 vol. %.
 110. The apparatus as claimed inclaim 109, wherein the porous agglomerates of carbon particles comprisea carbon black.